fix for when sub_slice cant be fetched
fix for when sub_slice cant be fetched

dbolser-ebi/ensembl-variation,Ensembl/ensembl-variation,willmclaren/ensembl-variation,dbolser-ebi/ensembl-variation,dbolser-ebi/ensembl-variation,Ensembl/ensembl-variation,dbolser/ensembl-variation,willmclaren/ensembl-variation,Ensembl/ensembl-variation,dbolser/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,dbolser/ensembl-variation,dbolser/ensembl-variation,willmclaren/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation